The present invention relates to a method for communication with no power feed and an exchange for realizing the method and, more particularly, to a method for enabling communication with no power feed, that is, without feeding a direct current (speech current) to a calling subscriber unit and an exchange for realizing this method.
In a conventional protocol for connecting a subscriber telephone and an exchange, a direct current loop is first formed by the switch circuit (hook switch) of the telephone via the telephone and the subscriber line interface circuit (SLIC) of the exchange, and a destination telephone number is then input through the telephone. The exchange identifies the calling subscriber telephone by detecting the direct current loop formed in the subscriber line interface circuit, and forms a speech path between the calling subscriber telephone and the called subscriber telephone so as to enable communication. When the exchange detects the disconnection of the direct current loop in the subscriber line interface circuit, the exchange regards it as the end of communication and disconnects the speech path between the calling subscriber telephone, which is connected to the subscriber line interface circuit, and the called subscriber telephone. A direct current loop is formed and a call is begun when the handset of the telephone is lifted (off-hook) and it is disconnected and the communication is finished when the handset is restored (on-hook). In addition, when the bell of the called subscriber telephone rings, a direct loop is also formed by the off-hook operation.
FIG. 13 shows the structure of a conventional exchange and FIG. 14 shows the entire structure of connection including a subscriber line interface circuit and telephones. In FIGS. 13 and 14, the reference numerals 1a.sub.1 to 1a.sub.n represent telephones, 2 an exchange, 2a.sub.1 to 2a.sub.n an subscriber line interface circuits connected to the respective telephones 1a.sub.1 to 1a.sub.n, 2b a PCM switch circuit constituted by a space switch, a time switch and the like, 2c a processor for controlling a calling process such as the connection/disconnection of the path by controlling the PCM switch circuit, 2d a main memory for storing various data, 2f a centralized supervisory equipment for discriminating the off-hook/on-hook of a telephone (connection/disconnection of a direct loop), identifying a dial number and transferring the information to the processor 2c, and 2g.sub.1, to 2g.sub.m trunk circuits.
In the telephone 1a.sub.1 in FIG. 14, the reference numeral 1.sub.1 denotes a hook switch, which is turned on when the handset of the telephone 1a.sub.1 is lifted so as to form a direct current loop (indicated by the broken line) for connecting the telephone 1a.sub.1 and the subscriber line interface circuit 2a.sub.1, and which is turned off when the hand set is restored to the original position so as to disconnect the direct current loop. The reference numeral 1.sub.2 represents a dial circuit for generating a dial pulse DP by disconnecting the direct current loop the number of times which corresponds to the dialed number, 1.sub.3 a speech circuit provided with a function of distributing the telephone cable to the signal lines of the transmitter and the receiver (not shown).
In the subscriber line interface circuit 2a.sub.1, the reference numeral 2.sub.1 represents a BSH circuit, and 2.sub.2 a CODEC (coder/decoder) for converting an analog speech signal (alternating current signal) into a PCM digital signal (A/D conversion) and converting a digital signal from a PCM highway 5 into an analog speech signal (D/A conversion). The BSH circuit 2.sub.1 is provided with a battery feed function for supplying a speech current to the telephone so as to operate the transmitter (microphone), etc., a supervisory function for supervising the state of the telephone so as to detect calling, answering, on-hook, dial pulse, etc., and a hybrid function. In the BSH circuit 2.sub.1, the reference numeral 2.sub.11 denotes a battery feed circuit (B) for supplying speech current, 2.sub.12 a supervision circuit (S) and 2.sub.13 a hybrid circuit (H). The transmission system on the telephone cable is a two-wire full duplex system for transmitting an electrical signal simultaneously in two ways through two wires.
When a direct current loop is formed via the hook switch 1.sub.1 of the telephone 1a.sub.1 and the battery feed circuit 2.sub.11 of the BSH circuit 2.sub.1, an alternating current signal as a speech signal of the telephone 1a.sub.1 is transmitted on the telephone cable superimposed with the direct loop current. In the subscriber line interface circuit 2a.sub.1, the alternating current signal passes through the hybrid circuit 2.sub.13 and it is converted into a digital signal by the CODEC 2.sub.2 and supplied to the PCM highway 5 in the form of a PCM signal. Thereafter, the PCM signal is processed by an exchange, it is input to the called subscriber telephone. The speech signal of the called subscriber telephone is input to the CODEC 2.sub.2 via the PCM switch circuit 2b (FIG. 13) and the PCM highway 5. The CODEC 2.sub.2 converts the digital signal into an analog signal and inputs the analog signal to the calling subscriber telephone superimposed with the direct loop current.
FIG. 15 is a flow chart of a conventional communication processing. When the handset of the telephone 1a.sub.1 is lifted so as to call, the off-hook switch 1.sub.1 is closed and a direct current loop is formed via the telephone 1a.sub.1 and the subscriber line interface circuit 2a.sub.1 (step 101). A destination telephone number (numeric information) is then transmitted in the form of a dial pulse signal (dial pulse system) or a PB signal (push-phone signal system) (step 102). In FIG. 15, the dial pulse system is shown.
The centralized supervisory equipment 2f of the exchange 2 is supervising the calling by scanning so as to find in which telephone the direct current loop is formed by operating the hook switch (step 201). When the telephone in which the direct loop current is formed is detected, the centralized supervisory equipment 2f reports the fact (SCN ON) to the processor 2c (step 202). When the processor 2c receives the report, it analyzes the information on the calling subscriber (e.g., whether or not the calling subscriber has paid the telephone bill) with reference to the calling subscriber data. If the processor 2c judges that communication is permissible, it connects a dial tone transmission circuit (not shown) to the calling subscriber telephone via the PCM switch circuit 2b so that the calling subscriber may hear the dial tone.
Thereafter, the centralized supervisory equipment 2f is in a numeric information receiving state (step 203). In this state, if the centralized supervisory equipment 2f receives the destination telephone number, it sequentially transmits the numerals to the processor 2c (step 204). The processor 2c disconnects the connection of the dial tone transmission circuit when it receives the first numeric information. When the processor 2c has received all the numerals of the telephone number, the processor 2c analyzes the information on the called subscriber (e.g., whether or not the called subscriber has paid the telephone bill, whether or not the telephone is occupied, and whether or not the telephone number is now in use). If the connection of the speech path is possible, the processor 2c starts to call the called subscriber and connects a ring back tone transmission circuit to the calling subscriber telephone so that the calling subscriber may hear the ring back tone (step 205).
If the processor 2c detects the off-hook of the called subscriber telephone (formation of a direct current loop), it stops calling the called subscriber telephone, disconnects the connection of the ring back tone transmission circuit, and forms a speech path between the calling subscriber telephone and the called subscriber telephone. Thereafter, the calling subscriber telephone and the called subscriber telephone are in a communication state (step 206). The exchange 2 continues the connection of the PCM highway 5 with the called subscriber telephones as long as the direct current loop is formed.
When the communication is finished and the handset is restored to the original position, the hook switch 1.sub.1 is turned off, and the direct current loop is disconnected (step 103). The telephone assumes the on-hook condition (step 104). The centralized supervisory equipment 2f assumes an on-hook supervisory state and supervises by scanning in which telephone the direct current loop has been disconnected. When the centralized supervisory equipment 2f detects the disconnection of the direct current loop of the telephone 1a.sub.1, it reports the fact (SCN OFF) to the processor 2c (step 207). When the processor 2c receives the report, it executes an on-hook processing such as the disconnection of the speech path between the telephones (step 208).
As described above, the direct current loop has not only (1) a function of providing a state signal indicating the off-hook/on-hook state of a telephone to the exchange (signalling function), but also (2) a function of supplying a speech current for operating the transmitter (microphone) of the telephone (battery function) and (3) a function of transmitting an alternating current signal (speech signal) superimposed with a direct current (alternating current transmitting function).
FIG. 16 shows the structure of the connection including a personal computer provided with a modulator-demodulator (MODEM) and a subscriber line interface circuit. The same reference numerals are provide for the elements which are the same as those shown in FIG. 14. The reference numeral 3a represents a personal computer, 4a a connecting apparatus containing a MODEM for connecting the personal computer 3a with the subscriber line interface circuit 2a, 4a.sub.1 a MODEM (modulator-demodulator) and 4a.sub.2 a switch for connecting/disconnecting a direct current loop. The symbol R denotes a direct current load resistance, and C a capacitor for disconnecting a direct current. The MODEM 4a.sub.1 transmits an alternating current signal (analog signal) in a voice band (300 to 3400 Hz) to a telephone cable after it is subjected to a predetermined modulation such as amplitude modulation, phase modulation, frequency modulation and QAM modulation with an I/O digital signal input from the personal computer 3a, and the MODEM 4a.sub.1 also demodulates an analog signal input from the telephone cable into a digital signal and inputs it to the personal computer 3a.
In order to start personal computer communication (electronic mail communication in an internet, etc.), a switch 4a.sub.2 is turned on under the control of the personal computer 3a so as to form a direct current loop in the same way as in a telephone, and then the switch is turned on and off in accordance with the destination telephone number so as to generate a dial pulse. The exchange connects the calling personal computer 3a with the called personal computer, and continues the connection of the PCM highway 5 with the called personal computers as long as the direct current loop is formed. Thereafter, the MODEM 4a.sub.1 modulates the analog signal with the digital signal input from the personal computer 3a and transmits it to the called personal computer. In order to end the personal computer communication, the switch 4a.sub.2 is turned off so as to disconnect the direct current loop in the same way as in a telephone.
Since the MODEM and the personal computer uses commercial power source (e.g., AC 100 V) as the power source, they need not use a direct current loop as a power source unlike the transmitter of a telephone. In addition, once the MODEM is connected to a line, it is possible to continue the alternating connection between the MODEM and the line even if the direct current loop is disconnected. For this reason, although the disconnection of a direct current loop during communication may be thinkable, since communication is started and finished in the similar protocol as in a telephone, and since an alternating current signal is transmitted and received through a subscriber line interface circuit, a direct current loop is continuously formed during personal computer communication.
Therefore, in the communication using a personal computer and a modem, the direct current loop has (1) a function of providing a state signal indicating the start/middle/end of communication to the exchange (signalling function), and (2) a function of transmitting and receiving an alternating current signal in the subscriber line interface circuit (alternating current transmitting function).
Although a dial pulse system is explained in the above, the same can be said with a push-phone signal system using a PB signal. In the case of a push-phone signal system, however, a PB receiver circuit (PB-REC) is necessary in the exchange.
As described above, when communication is made with a destination unit through a conventional exchange, a direct current loop is constantly formed during communication. The direct current loop is used for the detection of a state signal of a subscriber apparatus (telephone, personal computer, MODEM), as described above, and it is also used for the battery feeding in a telephone. A comparatively large current such as 20 to 100 mA is necessary as a speech current, and the subscriber line interface circuit of the exchange supplies the speech current by using -48 V. In the case of a MODEM and a personal computer, since they use a commercial power source, it is not necessary to use a direct current loop as a power source, but it is necessary to continuously form a direct current loop, as described above, so that about a current of 20 to 100 mA constantly flows through the direct current loop during communication.
Therefore, the power consumed by the exchange as a whole is very large. For example, if it is assumed that a current of 40 mA flows at 48 V per circuit, the electric power consumed is 1.92 W. In the exchange accommodating 10,000 circuits, the electric power consumed reaches 19,200 W (400 A). The conventional activity ratio of a subscriber telephone is about 0.15 erl (erlangs) on the average, so that the electric power consumed is reduced to 19,200 W.times.0.15=2880 W with the traffic taken into consideration. An exchange is generally designed on the basis of a traffic theory, and the power source and the heat of an exchange are designed on the basis of 0.15 erl. That is, they are designed on the assumption that the activity ratio of 1 erl never occurs, that is, on the assumption that all the subscribers never use the telephones simultaneously.
However, due to the spread of internets and electronic mails, the state in which subscribers uses their apparatuses for many hours has occurred. In personal computer communication or the like, computers are used for a much longer time in an off-hook state than telephones on which persons talk, and the activity ratio has a tendency to coming close to 1 erl. If it actually comes close to 1 erl, the electric power consumed by the exchange increases so much that it becomes a large problem with the exchange which is designed on the basis of the above-described traffic theory.